1. Field of the Invention
The present invention relates generally to the field of spread spectrum communications and, more particularly, to the extraction of soft symbols from an information channel in a sample stream, for code division multiple access (CDMA) RAKE integrated circuit receivers.
2. Description of Related Art
In spread spectrum communications systems, such as in CDMA systems, pseudorandom noise (PN) sequences are used to generate spread spectrum signals by increasing the bandwidth (i.e., spreading) of a baseband signal. A forward link waveform transmitted by the base station may be comprised of a pilot waveform and data waveforms. Each of the waveforms are received with the same relative phase and amplitude distortions introduced by the channel. The pilot waveform is an unmodulated PN sequence which aids in the demodulation process, as is well-known in the art as xe2x80x9cpilot-aided demodulation.xe2x80x9d Conventional pilot-aided demodulation methods typically include the steps of (i) demodulating a pilot waveform, (ii) estimating the relative phase and amplitude of the pilot waveform, (iii) correcting the phase of the data waveform using the estimated phase of the pilot waveform, and (iv) adjusting the weights of symbols used in maximal ratio combining in a RAKE receiver based on the estimated amplitude of the associated pilot waveform. Steps (iii) and (iv) above are performed as a xe2x80x9cdot productxe2x80x9d as is known in the art. Typically, Steps (i) through (iv) are performed using hardware. In some conventional methods, a controller having a central processing unit (CPU) and and/or a digital signal processor (DSP), with additional hardware blocks, may perform part of the above-described process.
FIG. 1 illustrates a conventional IS-95A or TIA/EIA-95-B forward link base station transmitter section 10 (prior art). A pilot channel 12 is generated that has no data. That is, the data is predetermined to be all xe2x80x9c0xe2x80x9d bits. The pilot channel is modulated, or covered with a Walsh code from Walsh code generator 14 at 1.2288 Mcps (mega chips per second). 64 orthogonal Walsh codes, each of 64 chips, are used in the IS-95A and TIA/EIA-95-B systems. Each channel is modulated with a unique Walsh code. Walsh code H0 is used to modulate the pilot channel.
Also depicted is a traffic or paging channel, which shall be referred to herein as an information channel. Data is input at one of a plurality of data rates from 9.6 kbps (kilobits per second) to 1.2 kbps. The data is encoded at encoder 16, at one bit per two code symbols, so that the output of the encoder 16 varies from 19.2 ksps (kilo symbols per second) to 2.4 ksps. Symbol repetition device 18 repeats the codes from 1 to 8 times to create a 19.2 ksps signal. Alternately stated, either 1, 2, 4, or 8 modulation symbols are created per code symbol. Then, the information channel is scrambled with a long code at the same 19.2 ksps rate. The information channel is covered with a different Wash code from that used to cover the pilot channel, code HT for example.
After being covered with a Walsh code, each channel is split into I and Q channels, and spread with I and Q channel PN sequences. A 90 degree phase shift is introduced by multiplying the I channel with a sin function, while the Q channel is multiplied with a corresponding cosine function. Then, the I and Q channels are summed into a QPSK channel. In the IS-95A and TIA/EIA-95-B standards, the same baseband symbols are assigned to both the I and Q channels. The composite waveform of all the QPSK channels, including pilot, synchronization, paging, and traffic channels is then up-converted in frequency (not shown) and transmitted.
FIG. 2 is a conventional IS-95A or TIA/EIA-95-B CDMA receiver (prior art). At the mobile station receiver 50, the transmitted signals are accepted as analog information, split into I and Q channels, and converted into a digital sample stream at A/D 52. Conventionally, a multi-finger RAKE is used to variably delay and amplify multipath delays in the sample stream, so that degradation due to fading can be minimized. Three demodulating fingers, demodulating finger 1 (54), demodulating finger 2 (56), and demodulating finger 3 (58) all receive the same I and Q sample stream. Each demodulating finger is assigned one of the sample stream multipath delays. PN codes and Walsh codes are generated with delays consistent with the multipath delays of the sample stream to be demodulated. The demodulated symbol streams from the multipaths are coherently combined in combiner 60 based on a maximal ratio combining (MRC) principle.
The IS-2000 standards propose, and future uses will include multiple information channels with a variety of symbol rates. A variety of symbol accumulation periods will be required in the process of demodulating these information channels. In IS-95A and TIA/EIA95-B standard communications, a symbol is conventionally spread with 64 PN chips at the transmitter. At the receiver, the symbol is recovered by despreading and accumulating the symbol over a period of 64 PN chips. The accumulated symbol is called a soft symbol. Conventionally, the soft symbol is corrected with respect to phase and weighted with respect to amplitude after accumulation, using the pilot waveform as a phase and amplitude reference. Although this method of correction is known to work when symbols are spread with 64 chips, longer accumulations before correction may result in degraded receiver performance.
Over the accumulation period, the phase of the received symbol is potentially changing. The dot product operation upon the symbol uses an average phase, to correct symbol phase. The rate of phase change is assumed to be slow relative to the soft symbol rate, so that a single phase correction can be used over the symbol period. However, if the accumulation period is so long that the phase at the beginning of the accumulation period becomes significantly out of phase with the phase at the end of the accumulation period, then a single phase correction for the entire accumulation period will not be effective, and would result in significant performance loss.
It would be advantageous if a CDMA RAKE receiver could be designed to maximize flexibility, permitting demodulating fingers to accumulate symbols in an information channel, regardless of the information channel symbol rate. Likewise, it would be desirable if the demodulating finger could be designed so that the various finger channels of the demodulating finger could operate at independent symbol rates.
It would also be advantageous if a CDMA RAKE receiver demodulating finger could be designed to be flexible enough to accumulate symbols for a wide variety of information channel symbol rates.
It would be advantageous if symbol accumulation could be performed to permit reasonably frequent phase corrections using pilot channel estimates.
It would be advantageous if the symbols could be partially accumulated before the dot product correction using the pilot estimate, and then further accumulated.
It would likewise be advantageous if the above-mentioned partial accumulation process could be accomplished over a wide range of symbol rates.
Accordingly, in spread spectrum communications, a demodulating finger integrated circuit is provided for extracting soft symbols from a sample stream including a plurality of information channels. The demodulating finger comprises a plurality of finger channels, and each finger channel includes a first accumulator, a dot product unit, and a variable accumulator. The first accumulator accepts an uncovered sample stream and accumulates the uncovered sample stream to supply partial I and Q accumulations to the dot product unit. The first accumulator supplies partial I and Q accumulations at a rate of one partial I and Q accumulation per four PN chips. The dot product unit accepts partial I and Q accumulations and a pilot estimate. In response, the dot product unit provides partial symbols. The variable accumulator, connected to the dot product unit output, accepts accumulation period commands and supplies soft symbols at a rate responsive to the accumulation period commands. The variable accumulator permits the demodulating finger to be used in demodulating information channels at various information channel symbol rates. Specifically, the variable accumulator supplies soft symbols with a symbol period in the range between 4 and 2048 PN chips.
A method is also provided for extracting soft symbols from a sample stream including a plurality of information channels, using an integrated circuit including a demodulating finger. The method comprises: uncovering a sample stream to supply an uncovered sample stream; accumulating the uncovered sample stream to supply partial I and Q accumulations at a rate of one partial I and Q accumulation per four PN chips; performing a dot product operation upon the partial I and Q accumulations; in response to performing the dot product operation upon the partial I and Q accumulations, supplying scalar partial symbols; selecting an accumulation period to accumulate partial symbols at a symbol period in the range between 4 and 2048 PN chips; and, supplying soft symbols at a rate responsive to the selected accumulation period.